It has long been known to obtain color images by contact reaction between an electron-donating or proton-accepting colorless organic compound (hereinafter referred to as a color former) and an electron-accepting or proton-donating solid acid (hereinafter referred to as a color developer). Examples of specifically utilizing this phenomenon are pressure-sensitive copying papers (for examples, U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250 and 3,672,935), heat-sensitive recording papers (for example, Japanese Patent Publication Nos. 4160/68, 7600/68, 14039/70, U.S. Pat. No. 2,939,009), and so forth.
Furthermore, a printing method is known which comprises supplying a color former-containing ink to a color developer-coated sheet to obtain colored images (German patent OLS No. 1,939,962).
The color developer has the above-described properties and examples thereof include acid clays, phenol resins, aromatic carboxylic acid metal salts, etc.
As the color former, there have been used compounds which are usually colorless, which possess a partial skeleton of lactone, lactam, sultone, spyropyran, ester, or amide, and which undergo ring cleavage or splitting of said partial skeleton when contacted with a color developer. Examples include triarylmethane compounds such as crystal violet lactone, xanthene compounds such as rhodamine B anilinolactam, spiro compounds such as 3-methyl-spiro-dinaphthopyran, thiazine compounds such as benzoyl leucomethylene blue, and leucoauramine compounds such as N-(o-chlorophenyl)leucoauramine.
However, colored products formed by the reaction between a color former of a triarylmethane compound, xanthene compound, spiro compound or the like and an acid clay have poor durability, and when contacted with water or an organic solvent undergo a reduction in density or the disappearance of color. Of organic solvents, polar solvents such as alcohols (e.g., ethanol, butanol, propylene glycol, glycerin, etc.), amines (e.g., monoethanolamine, triethanolamine, etc.), and esters and ethers (e.g., ethyl acetate, diethylene glycol, monomethyl ether, dioctyl phthalate, dibutyl phthalate, etc.) exert particularly strong influences on colored products when the organic solvents are contacted, directly or as a vapor, with the colored products, resulting in a reduction in color density or the total disappearance of color.
The inventors have formerly proposed to use benzhydrylsulfone type color formers so as to overcome the above-described defects as described in Japanese Patent Application No. 57593/81. However, when a benzhydrylsulfone type color former is encapsulated and coated on a base paper to prepare a coated paper, the resulting paper has the defect that the coated surface is stained bluish.
Known microencapsulating processes include a coacervation process, an interfacial polymerization process, an in situ polymerization process, etc.
In employing the coacervation process using gelatin, the above-described defect is not so serious. However, in the case of employing the interfacial polymerization process, i.e., causing a polymerization reaction at the interface of a core material comprising a hydrophobic monomer and a hydrophilic monomer, and the in situ polymerization microencapsulation process, i.e., forming a wall from outside (hydrophilic phase) or inside (hydrophobic phase) of a core material, such as urea-formaldehyde resin-walled capsules and melamine-formaldehyde resin-walled capsules, the above-described "stain" is liable to appear.